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The twilight zone

THE ancients had no problems distinguishing stars and planets: stars are
fixed in the firmament, planets move relative to them. Ours is a more
sophisticated age. We don’t know what the difference is.

To find the answer, we must enter the twilight world of the brown dwarf.
Brown dwarfs are hardly stars at all. Too lightweight to trigger hydrogen fusion
in their cores, they glow so feebly that astronomers found direct evidence for
their existence only four years ago. At about the same time, astronomers began
to discover giant planets orbiting other stars.

The smallest brown dwarfs are almost indistinguishable from the giant
planets. No one can say for certain which objects are planets and which are
stars. This could be crucial not only to understanding the nature of the
mysterious “dark matter” that makes up 90 per of the mass of the Universe, but
even in the search for extraterrestrial life.

It’s the desire to find out whether or not we are alone in the Universe that
has driven astronomers to scour the heavens for evidence of planets around other
stars. But if some of the giant planets they have found are really brown dwarfs,
planetary systems such as our own might be much rarer than we think. What’s
more, until we learn the differences between the smallest stars and the largest
planets, and how they form, it will be hard to recognise solar systems that
might hold Earth-like planets capable of supporting life.

Some of the latest research hints at a true demarcation between tiny stars
and giant planets, something that would go a long way towards solving …